Systems for generating light with a variable color are already known. By way of example, reference is made to U.S. Pat. No. 5,384,519, which describes a system with three individual light sources, each light source producing light with a specific color, the three specific colors being mutually different. The light produced by the system as a whole contains a mixture of the light produced by three individual light sources, and the color of the light mixture is a mixture of the three specific colors. For varying the color of the light mixture, the relative light intensities of the three individual light sources can be set at a certain ratio.
Each of the light sources has a nominal output power, and each of the light sources can be dimmed such that the actual light output power of such light source is lower than the nominal output power. Setting the relative light intensities of the three individual light sources is done by adequately setting the respective dim factors of the three light sources.
Having set the color of the light mixture as desired, the output intensity of the system as a whole can be varied while keeping the color constant. To this end, the light intensities of the three individual light sources are varied, such that the ratio of the relative light intensities is maintained constant in order to keep the color constant. A problem in this respect is that the light intensity of each light source can only be varied within a certain range defined by a minimum intensity level and a maximum intensity level, which maximum intensity level typically corresponds to the nominal intensity. The maximum output intensity of the system as a whole is reached when the light source having the highest relative intensity reaches its maximum intensity level: a further increase in intensity is not possible for this light source. The minimum output intensity of the system as a whole is reached when the light source having the lowest relative intensity reaches its minimum intensity level: a further decrease in intensity is not possible for this light source. The variable intensity range is largest for colors where the light intensities of the three individual light sources are substantially equal. The variable intensity range is lower for colors where the light intensities of the three individual light sources differ greatly. The variable intensity range is lowest for colors close to the outer edges of the color gamut.
In said document U.S. Pat. No. 5,384,519, a system is disclosed for obtaining a specific desired output color at a certain desired dim level. Corresponding control signals for the three light sources are taken from a memory, and the three light sources are controlled by the three corresponding control signals as read from memory. Then, the actual output light is measured, and it is checked whether the actual output light is in conformity with the settings. If it is found that a first one of the light sources produces not enough light, the control signals for the other two light sources are adapted such that the light outputs of the other two light sources are reduced, in such a manner that the mixture has the desired color; however, a consequence is then that the intensity of the mixture light is less than expected.
If one of said other two light sources is at its minimum intensity, reducing the light output of this one light source is not possible. Then, the control signal for the said first one of the light sources is adapted such that the light output of this first light source is increased, and the control signals for the other two light sources are adapted, such that the desired color ratio is obtained and hence the mixture has the desired color; however, a consequence is then that the intensity of the mixture light is higher than expected. Thus, this publication aims at keeping the color point constant but at the expense of sacrificing the light intensity.
The present invention aims to solve or at least reduce the above problems. More particularly, the present invention aims to provide a light generating system which can be dimmed over an extended dim range while maintaining the color.